Light energy, including but by no means limited to laser light energy, has been used in medicine and surgery for many years. Different wavelengths of light interact differently with tissue, so tissue effects are wavelength-dependent. Tissue effects are also dependent upon the amount of energy, expressed as joules (J). Energy is the product of power, in watts, and duration of exposure, in seconds. The power density (power divided by area) applied to the tissue is a variable frequently controlled by the physician. Power density is, in turn, a function of the distribution pattern of light energy applied to the tissue from a light source or light-emitting member of an optical delivery system.
Lasers in particular are used in many different types of medical procedures. Different lasers cause different tissue effects, depending upon the wavelengths of the laser emission. Among the types of lasers used in laser medicine are the CO.sub.2 laser, the KTP laser and the neodymium:YAG laser. In addition to laser light sources, which are referred to as "coherent," non-laser or "incoherent" light sources may also be used for medical and surgical procedures.
In order to get the light energy from the light source to the tissue to be treated, it is desirable to have a delivery system between the light source and the operative site. Such delivery systems as used in medicine and, in particular, in surgery can be broadly divided into those which either contact or do not contact tissue to be treated. In non-contact delivery systems, the distal end of the delivery system does not touch the tissue but, instead, uses a fiber optic or other light guide means to conduct light energy from a light source to a location adjacent, but not touching, the tissue. The light energy passes from the distal end of the delivery system through a gas or a liquid before reaching the tissue. In addition to non-contact procedures, techniques and devices have been developed in which the distal end of the delivery system comes into physical contact with the tissue.
In both contact and non-contact procedures, different tissue effects can be achieved by using different output energy distributions from the delivery system. One way of varying the output energy distribution is to vary the size and shape of the distal end of the delivery system. Another is to provide structure or coatings on the distal end which absorb, scatter, or both absorb and scatter, energy from the light source in a controlled and predictable manner before it reaches the tissue.
U.S. Pat. No. 4,693,244 discloses a medical and surgical laser probe in which the distal end of the probe is tapered so as to emit laser radiation from the tip end face of the tapered portion without leaking it out from the tapered portion. Such a structure is highly effective for incising tissue because it provides a high power density at the end face of the tapered portion. However, the laser probe described in the '244 patent is also useful in other procedures where a more diffuse energy density is desired. Thus, in an alternate embodiment of the invention disclosed in the '244 patent, the end face has formed on it a curved surface, which includes numerous fine bubbles which diffuse laser radiation emitted from the end face. The diffusion pattern can be varied by varying the curvature of the surface. Consequently, a delivery system incorporating a curved surface with numerous fine bubbles therein can be used for a number of surgical procedures in which scattered laser radiation from the probe is desirable.
In the embodiment of the '244 patent which uses a curved surface with numerous fine bubbles therein, the larger the curvature the greater the diffusion angle of the laser radiation. Also, ideally the bubbles should be of a uniform shape and size. If the bubbles are too large, the laser energy will be reflected in undesirable directions (e.g., back toward the fiber optic). The number and distribution of bubbles should be such that the tip is translucent, with a light transmission of between 20 and 50 percent. The '244 patent points out that if there are too many bubbles, and consequently the transmission of laser radiation is decreased, too much of the laser energy may be lost to heat. Thus, according to the '244 patent, it is possible that, in use, the temperature of the tip may exceed the maximum operating temperature for the material from which the tip is manufactured. This can cause the tip to deform and alter its physical shape and characteristics.
Currently, contact probes are manufactured primarily from one (or both) of two materials, sapphire (Al.sub.2 O.sub.3) and/or quartz. Sapphire is preferred in many cases because it has a useful working temperature up to 1800.degree. C., with a melting point of 2040.degree. C. In contrast, the maximum useful working temperature for quartz is approximately 900.degree. C., and its melting point is only 1600.degree. C. Laser surgical procedures can result in the tip end reaching temperatures in excess of 1000.degree. C., which can cause quartz tip ends to deform and alter their physical shape. A change in the shape of the tip end can change the power density at the tip end and the power distribution profile, which alters the clinical effect of the tip. Moreover, sapphire can be used as a mechanical tool in addition to a laser energy transmission device. Sapphire has a Young's modulus of 5.3.times.10.sup.7 psi, whereas the Young's modulus of fused silica is only 1.02.times.10.sup.7 psi. Because of this, a tip made from fused silica used in a mechanical mode exhibits a much greater potential for breakage.
One the other hand, sapphire is more susceptible to thermal shock than is fused silica. Thermal shock can occur when the temperature of a material changes very rapidly. Such a rapid temperature change induces physical changes inside the tip end which can cause a sapphire tip to check or shatter. One indicator of a material's resistance to thermal shock is its coefficient of thermal expansion. For sapphire, this coefficient is 7.7.times.10.sup.-6 cm/cm.degree.C., at 40.degree. C., whereas for fused silica this coefficient is 0.55.times.10.sup.-6 cm/cm.degree.C. at 40.degree. C. As stated above, laser surgical procedures can result in extremely high tip end temperatures, which can lead to thermal shock problems. Hence, there may be some procedures where fused silica is an acceptable, or even the preferred alternative to, sapphire.
In addition to fused silica and sapphire, tips fabricated from resins have been proposed. Resin tips are relatively easy to fabricate into structures of various shapes and which contain light absorbing and/or light reflecting particles. The type and distribution of particles, as well as the shape of the tip, affect the output energy distribution of those tips. Resin tips, however, are limited to low-temperature uses, since the temperatures at which resins will plastically deform and melt are quite low in comparison to fused silica and sapphire. Resin tips tend also to be much softer than fused silica and sapphire, and therefore lack the mechanical strength required of a tip in some procedures. Also, some resin tips, when overheated, can burn and emit toxic fumes.
The present invention aims to overcome the disadvantages of prior delivery systems while retaining their respective advantages.